A satellite communications system may be conceptually sub-divided into a space segment, a user segment, and a ground (terrestrial) segment.
As an example, in one type of mobile satellite communication system the satellite segment includes a number of satellites in orbit. The satellites are distributed in orbital planes that are inclined with respect to the equator. Preferably, at least two satellites are in view at any given time from a particular user location between about 70 degree south latitude and about 70 degree north latitude.
The user segment may include a plurality of types of user terminals that are adapted for communication with the satellites. The user terminals include a plurality of different types of fixed and mobile user terminals including, but not limited to, handheld mobile radio-telephones, vehicle mounted mobile radio-telephones, paging/messaging-type devices, and fixed radio-telephones. The user terminals are preferably provided with antennas for bi-directional communication via one or more of the satellites. Communication between the user terminals and the satellites are conveyed by forward (from satellite to user terminal) and return (user terminal to satellite) service links. The user terminals may also be dual use devices that include circuitry for also communicating in a conventional manner with a terrestrial communications system.
The ground segment includes at least one but generally a number of system gateways that communicate with the satellites via feeder links. Communication signals between the system gateways and the satellites are conveyed by forward (gateway to satellite) and return (satellite to gateway) feeder links. Feeder links also convey commands from the system gateways to the satellites and telemetry information from the satellites to the system gateways. The system gateways further function as the principle connecting points to couple the communications payload or transponders of the satellites to a network infrastructure. The network infrastructure may include the PSTN and other existing telephone systems, Public Land Mobile Network (PLMN) gateways, local telephone exchanges such as regional public telephone networks (RPTN) or other local telephone service providers, domestic long distance networks, international networks, private networks and other RPTNs. It should be noted that the network infrastructure may further include other types of networks, such as a wireless network, X.25, the Internet, TCP/IP, ATM, etc. The satellite communication system thus operates to provide bi-directional voice and/or data communication between the user segment and the network infrastructure.
The ground segment of a low earth orbit satellite communication system may further include virtual gateways. Virtual gateways function under the control of system gateways which assign tasks for the virtual gateways to perform in the setup, call management, and call tear down procedures. A virtual gateway may also manage the satellite system resources that are allocated to it on a part time, as-required basis. Under the direction of a system gateway, a virtual gateway may function as a local gateway for the duration of the setup, call, and call tear down time and then may relinquish its authority and control of the system resources after these functions have been performed. While performing its assigned functions, the virtual gateway may reallocate its assigned resources one or more times, as required. Of course, it is possible that there are many calls being simultaneously handled by the virtual gateway, and indeed, in some installations it is possible that the virtual gateway is active 100% of the time.
In some cases a virtual gateway and its users may form a closed network. An example is a terrestrial wireless local loop. There may be many virtual gateways, that is, many disconnected closed-networks, distributed over the service area of a system gateway. Communication between user terminals of the system (i.e. system users) and users associated with the virtual gateway may be accomplished without utilizing the network infrastructure segment. Communication among the virtual gateways and a system gateway may be made in a manner similar to that for the communication between the system gateway and the system user terminals; that is, via the conventional feeder and service links. Communication among the distributed virtual gateways and the system user terminals may be accomplished via the service links of the satellites. In addition, other communication routes such as forward feeder to return feeder link and inter-satellite links are also possible.
Reference in this regard can be had, by example, to U.S. Pat. No. 5,884,142, issued Mar. 16, 1999, entitled “Low Earth Orbit Distributed Gateway Communications System”, by Robert A. Wiedeman and Paul A. Monte. The disclosure of this issued patent is incorporated by reference in its entirety insofar as it does not conflict with the teachings of the present invention.
The signals included in a particular link are typically composite signals, grouped into channels within the allocated frequency band of the link. The channels within a link may be further grouped into a number of channel blocks.
Spread-spectrum techniques may be used for code-division multiplexing and demultiplexing channel blocks in a link within a satellite system. Orthogonal chip-coded waveforms may be used for frequency spreading the channel blocks at an appropriate intermediate frequency and then the channel blocks may be upconverted to the link frequency. As an example, in a forward feeder link (from a system gateway to a satellite), a channel block may be composed of numerous code division multiple access channels, possibly of various bandwidths, that are frequency division multiplexed together. The same allocated frequency spectrum of the forward feeder link may be reused by each spread channel block in the forward feeder link. Upon reception, the satellite may de-multiplex the spread channel blocks using code division techniques. The satellite may further generate inputs to a beam forming network of a transmitting multi-beam antenna for a forward service link (from the satellite to users). In this example, the same allocated frequency band of the forward service link may be reused by each antenna beam. As a further example, the same code-division multiplexing scheme may be used in a return feeder link from a satellite to a system gateway. The signals received from a return service link may be code-division multiplexed and the allocated frequency spectrum of the return feeder link may be reused by each spread channel block transmitted to the system gateway from the satellite. Upon receipt by the system gateway, the return feeder link may be code-division de-multiplexed and the de-multiplexed channel blocks may be utilized in a conventional manner.
The advantages of this example are that the allocated frequency bands are reused by each transmitted and received channel block. The power spectrum of each transmitted and received channel block is uniformly flat over the mid-band of the allocated frequency band resulting in better spectral and power efficiency. Also, using this scheme, the satellite requires no bulky input demultiplexing filters.
Reference in this regard can be had, by example, to U.S. patent application Ser. No. 09/504,130, filed Feb. 15, 2000, entitled “Feeder Link Code-Division Multiplexing and De-multiplexing for a Satellite Communication System”, by Wing-po Yung and Paul A. Monte, now abandoned. The disclosure of this patent application is incorporated by reference in its entirety insofar as it does not conflict with the teachings of the present invention.